The hematopoietic stem cell (HSC) through proliferation and differentiation gives rise to all of the cells in the hematopoietic system. Pluripotent HSCs are considered to be ideal candidates for disease therapy and they serve as attractive target cells for delivery of genes and gene products. It is desirable to have access to large amounts of such HSCs; unfortunately, hematopoietic stem cells are present in extremely low numbers in tissues where they are found, such as bone marrow and cord blood. Therefore, there is a need for improved methods of ex vivo cell culture systems capable of expanding hematopoietic cells, while maintaining stem cell pluripotency. It is also important to be able to readily identify cells retaining such pluripotency.
Difficulties in ex vivo expansion of HSCs have greatly hampered their clinical utility as well as studies of their biological properties. The identification of new protein factors that can stimulate the self-renewal or prevent their apoptosis is an essential way to increase the number of HSCs, including long term HSCs (LT-HSCs) in culture.